Gas turbine engines operate at extremely high temperatures for increased efficiency. Stationary vanes, disposed between rings of moving blades within the turbine section of the engine direct and stabilize high temperature gas flow from one stage of moving blades to the next. Direct exposure to this high temperature gas, however, detrimentally affects the vanes and blades by causing component distortion and even melting in extreme cases.
Internal cooling techniques have been developed to keep the temperature of the blades and vanes within design limits while operating at high temperatures. For example, the outer surface of engine components are typically cooled with high pressure cooling air from the compressor section of the engine. Film cooling has proven to be an effective means of utilizing this cooling air. In this method, a layer of cool air is flowed between the high temperature gases and the external surfaces of the engine components. The layer of cooling air is formed by passing the cooling air through a series of small holes in the component which are formed in a predetermined pattern. The resulting film of air reduces component surface temperature thereby deterring component distortion. Engine efficiency is also increased because higher turbine inlet temperature ranges are possible.
It is well known in the art that film cooling effectiveness can be increased by using diffusion holes which have a conical portion and an enlarged opening at the surface of the component, as shown in FIG. 1. The shaping of the holes to diffuse air before it enters the boundary layer of the component broadens the spread of air downstream of the hole and thus, increases cooling effectiveness. In comparison, cylindrical shaped holes create a localized region downstream of the hole where cooling effectiveness is great and there is little spreading effect away from this region. Although high quality diffusion holes provide superior performance, they are both costly and difficult to form.
In the prior art, many attempts have been made to form cost effective, high quality cooling holes in gas turbine engine components. For example, laser drilling has been used to produce holes on the leading and trailing edges of vanes and blades. It is difficult, however, to produce shaped holes with this technique. This is a significant limitation because the geometry of the holes partially determines the effectiveness of cooling.
Electrochemical machining (ECM) is another option for producing diffusion holes. This process, however, requires high setup and tooling costs and has high capital equipment costs. In addition, the electrolyte in this process, typically an oxidant such as sodium nitrate or sodium chlorate, is a health and safety hazard and the process by-products are often classified as hazardous waste.
Another method, electrical discharge machining (EDM), can also be used to form shaped diffusion holes in engine components. EDM is a well known process for producing holes or other openings in metals. It uses current discharges to erode metal. For example, by pulsing a direct current between a positively charged work piece (anode) and an electrode (cathode), a spark discharge may be produced. The current occurs when the potential difference between the electrode and the work piece, which both contact a dielectric fluid, is great enough to breakdown the dielectric fluid and produce an electrically conductive channel. Upon application of a voltage, a current flow results with enough heat energy to melt and erode the work piece. This process has application in the machining of small, deep, odd-shaped holes which are cumbersome, if not impossible, to produce by other means.
An EDM method for producing diffusion holes in engine components uses a copper electrode which is manufactured in a three-dimensional shape by stamping and coining. The prior art one-piece electrode consists of at least one small diameter elongated end which produces the cooling air metering section. The elongated end is connected to a three-dimensional diffuser shaped portion which produces a diffuser area for the metering section. The electrode produces a similar shaped hole, with allowance for electrode overburn and EDM electrode erosion.
Although the above EDM method is successful, limitations exist. For example, copper electrodes have a significant length/depth limit and a significant lower diameter limit for the holes due to the low melting point of copper. The hole depth limit for the above described copper electrode is about 0.250 inches maximum at a minimum diameter limit of about 0.014 inches. These values are design norms based on production experience for a shaped copper electrode.
Accordingly, what is needed is an EDM electrode which can produce high quality, deeper shaped holes with smaller metering diameters than previously possible.
The objects of the present invention are to (1) provide an economical electrode for use in an EDM device for producing high quality, deeper diffusion holes with smaller diameters than previously possible; and (2) provide a cost efficient method for forming deep, smaller diameter diffusion holes in articles, such as gas turbine engine components, than previously possible.
These objects and other features and advantages of the invention will be apparent from the following disclosure and description of the Best Mode, read in conjunction with the drawings.